Project Summary/Abstract Myotonic dystrophy (DM) is one of the most common forms of muscular dystrophy, affecting 1 in 8,000 people. It is common for DM patients to have digestive symptoms, often involving altered gut movement, including slowed stomach emptying, slowed intestinal movement, abdominal pain, diarrhea, and constipation. Intestinal bacteria play important roles in digestion, and many DM patients have overgrowth of these bacteria. A quarter of DM patients describe digestive symptoms as the most disabling aspect of the disease, yet these symptoms are poorly understood. The proposed work uses zebrafish to study DM because digestive symptoms can be observed directly in live, transparent larvae, and because zebrafish produce large numbers of offspring that can be used to screen for drugs to improve DM symptoms. The most common form of DM is caused by expression of RNA containing long repeats of CUG nucleotides, which bind to and sequester the MBNL family of proteins and prevent them from regulating an important molecular process called alternative splicing. Zebrafish have been generated that mimic the loss of MBNL protein function in DM by mutation of mbnl genes, and these fish show some of the same alternative splicing changes that are seen in human disease. Fish are also being made that mimic DM in another way, by expressing CUG repeat RNA. Using the new DM model fish, the first goal is to determine whether they have DM-like symptoms and to identify molecular changes that contribute to these symptoms. The next goal is to measure changes in gut movement in live zebrafish DM models using microscopy, to determine whether bacteria contribute to these changes, and to determine which tissue types are important for these symptoms. Finally, a small-scale test of candidate drugs to ameliorate the DM-like symptoms of model fish will be performed. This will serve as a proof-of-principle for future large-scale screening for drugs that improve DM-like symptoms in fish with the long term goal of identifying drugs to reduce or eliminate symptoms in DM patients. Overall, these studies will provide important insight into the poorly understood digestive symptoms of DM and the molecular mechanisms behind them. They will also generate novel animal model tools that will be invaluable for studying other aspects of DM pathology and for screening for drugs to improve the lives of individuals with DM. !